A known fuel injection system may include a plurality of unit pumps, each delivering fuel at high pressure to a respective, separate high pressure fuel line. Each unit pump typically includes a tappet that is driven by a cam to impart drive to a plunger, thereby causing the plunger to reciprocate, in turn, pressurizing fuel within a pumping chamber of the unit. Each unit pump is arranged to supply fuel to an injection nozzle of a respective dedicated injector so as to facilitate delivery of fuel to an associated cylinder of the engine. In such fuel injection systems, it is, therefore, necessary to provide each engine cylinder with a set of separate pump components, each consisting of a cam, a tappet, a unit pump, a high pressure line and an injector, wherein the cams for each set of pump components, typically, are carried on a common camshaft.
The cam of each unit pump is suitably mounted upon and driven by a camshaft that also carries the cams that control engine valve timing. In that case, the unit pumps are spaced in line along the axis of the camshaft, with a drive end of each unit pump co-operating with a lobe or lobes of its associated cam and the injection nozzle end of each unit pump being arranged to deliver fuel to the associated engine cylinder. Typically, the camshaft has at least three lobes associated with each engine cylinder; one for driving the associated pumping plunger and the other two for controlling engine valve timing. The camshaft extends through the crankcase of the engine, which is provided with pockets or bores for accommodating the unit pumps. The unit pumps are all therefore effectively housed within a common engine housing. For the purpose of this specification, any reference to the camshaft “carrying” a cam is intended to include carrying or mounting a separate cam upon the camshaft, or integrally forming the cam with the camshaft.
Fuel injection pumps are known wherein a plurality of pumping elements or plungers are incorporated within a unitary housing. Such arrangements are commonly referred to as ‘in-line’ pump arrangements, as the pumping elements are mounted in a line parallel to the axis of a camshaft that drives the plungers. Such systems require a set of tappets and a set of pumping plungers, one tappet and one plunger for each engine cylinder, with each tappet and its associated plunger being arranged within the associated unitary housing. As in unit pump arrangements, each pumping element has an associated pumping chamber that is connected to its associated injector through a separate high pressure fuel line. As a separate pumping element is provided for each engine cylinder, again, the costs of such systems are relatively high.
Common rail fuel injection systems are also known and typically include a common rail fuel pump having a plurality of pumping plungers driven by a common eccentric cam surface. The cam surface is rotatable by means of a drive shaft, and such pumps may include three or more plungers radially spaced around the drive shaft. The cam surface of the pump co-operates with all of the plungers to cause phased, cyclical movement of the plungers and, hence, pressurization of fuel within their associated pumping chambers. That pressurized fuel is fed to a common rail accumulator volume that in turn supplies fuel to all of the injectors of the system. Whilst common rail systems such as this avoid the need for one pumping element per engine cylinder, such radial pump arrangements are incompatible with existing in-line cam drive arrangements such as that described previously and hence a totally different engine layout is required to accommodate the system.
The machining and assembly line facilities for the manufacture of engine installations having unit pump fuel injection are well established, and engine installations that can accommodate unit pump fuel injection systems are widely used. It is therefore desirable to permit continued use of such existing production facilities and engine installations. However, it is also desirable to avoid or at least to mitigate several disadvantages associated with fuel injection systems having a plurality of unit pumps.